Project Summary/Abstract Injuries or diseases that kill retinal neurons and receptors block vision at it source. The resulting blindness is permanent. The inability to repair in the retina is a hallmark of the human central nervous system; neurons that die are not replaced and functions that are lost are not recovered. This bleak outcome is a driving force for research on neural stem cells and the field of regeneration biology. The long-term objective of this project is to elucidate the mechanisms that regulate the birth, death and regeneration of neurons and photoreceptors in the vertebrate retina. This program of research utilizes the zebrafish, because the fish retina is the only vertebrate CNS tissue where intrinsic stem cells can regenerate a single neuronal type that integrates into an existing circuit or can regenerate all cell types that completely restore the original tissue. Thus, investigating stem cell-based neuronal and photoreceptor regeneration in the teleost retina will advance our knowledge of mechanisms that govern the ability of intrinsic stem and progenitor cells to restore neural circuits in the injured brain. The resulting knowledge will guide the therpeutic use of transplanted stem and progenitor cells to treat retinal injuries, blindness and disease. Three Specific Aims are proposed, each directed toward revealing mechanisms that regulate the genesis and regeneration of photoreceptors. Specific Aim 1 will investigate the function of the bHLH transcription factor, NeuroD, in both the developing and regenerating retina. Specific Aim 2 will test the hypothesis that microglia play a neurogenic role during photoreceptor regeneration. Specific aim 3 will test the hypothesis that the soluble growth factor, midkine-a, controls aspects of retinal development in larval zebrafish and photoreceptor regeneration in adults. Together these specific aims represent a focused and integrated research program to test specific hypotheses of the biology and regulation of retinal development and neuronal regeneration in the vertebrate nervous system.